1. Field of the Invention
The present invention relates to compaction methods and compaction apparatus and in particular but not exclusively to methods and apparatus for compacting (i.e. compressing) waste material. Other applications for the invention include the compaction of materials used in farming and the food industry. These are not necessarily waste materials.
2. Description of the Related Art
Various types of material, including waste material such as litter and discarded packaging material, are bulky but not heavy. It is therefore desirable in certain circumstances to compact this material to reduce its volume in order to reduce transport costs or the storage space required.
Compacting apparatus is known which has conveying means operable to convey material along a path, which material is compacted as it moves along the path. Thus, the conveying means is arranged so that the density of material passing through the conveyor is relatively low at the beginning of the path and relatively high at the end of the path.
Such compaction can be achieved by using a screw conveyor located in a passage. Screw conveyors of constant pitch are generally used but in some arrangements a screw conveyor having a pitch which is relatively large at the beginning of the path and relatively small at the end of the path is used.
These known apparatus sometimes include a tapered portion near or at the discharge end of the path for further compaction. The cross-sectional area is thus relatively large at the entry to the tapered portion and relatively small at the exit thereof.
However, the known apparatus suffer from the problem that optimal performance can only be achieved for one set of operation conditions such as temperature, volume of waste fed into compactor and type and density of waste material. As will be appreciated, most waste compactors would in practice be exposed to a range of values for each condition. For example, a waste compactor which is situated outdoors would be subjected to the extremes of summer and winter temperatures. Likewise the uniformity of volume and type of waste cannot be guaranteed. For example an increase in the percentage of fatty or oily substances in the waste material can have an adverse effect on the performance of the compactor. Indeed in experiments carried out, the compaction achieved dramatically decreases especially for very slippery materials. The inability to deal with changes in conditions means that compactors will at one extreme not always compact material to the desired degree if at all and, at the other extreme, will have a tendency to jam.
Prior art is known which attempts to address at least certain aspects of this problem. For example, a resiliently biased trap door can be provided at the end of the conveying means which opens only when the pressure on the door exceeds a certain value. In theory, this allows waste material to accumulate so that variations in the volume of waste material do not affect the performance of the compactor. However, in practice, such doors remain partially open most of the time which leads to unsatisfactory results since maximum compaction is not achieved. Furthermore trap doors exert a sidways force which tends to encourage the break up of the compacted waste material which in turn can cause difficulties with the packaging and/or disposal of the material as well as increasing the volume of the compacted waste. In any case such compactors are not suited to applications requiring a relatively high degree of compaction as provided in accordance with embodiments of this invention.
Another problem from which the prior art compactors suffer is that the screw conveyor is generally very heavy in order to be sufficiently heavy duty to compress and compact waste or other material without itself being damaged. In particular, the thickness of the screw conveyor flight is dictated by the maximum force to which any part of the flight is subjected. The weight of the screw conveyor can lead to problems with providing sufficient support therefor.
Another problem associated with the weight of the screw conveyor concerns the provision of sufficient support for the conveyor. In the prior art, such screw conveyors are usually supported by fixed bearings at one end of the screw conveyor so that the longitudinal axis of the conveyor is immovable. The weight of the screw conveyor requires that the bearings be large and support the screw conveyor along a significant proportion of the length thereof. A mechanical bearing cannot be successfully used to support the outlet end of the screw conveyor since this would partially block the compactor outlet and interfere with the flow of compacted material which is of course undesirable.
Yet another problem with known screw conveyors is that they have a tendency to jam when a relatively incompressible object is fed into the conveyor. Under such circumstances, known screw conveyors attempt to continue to rotate with the motor applying an increased torque. This puts a strain on the motor and often is not sufficient to unjam the apparatus. The apparatus then will require manual attention to remove the blockage.